Curcumin [1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione]
is the major yellow pigment of turmeric, a commonly used spice, derived from the rhizome of the herb Curcuma longa Linn. In the Indian subcontinent and Southeast Asia, turmeric has traditionally been used as a treatment for inflammation, skin wounds, and tumors. Clinical activity of curcumin is yet to be confirmed; however, in preclinical animal models, curcumin has shown cancer chemo preventive, antineoplastic and anti-inflammatory properties (Kelloff, G. I., et al, J. Cell Biochem., 1996, 265:54-71). Especially interesting is its ability to prevent the formation of carcinogen-induced intestinal premalignant lesions and malignancies in rat (Rao, C. V. et al, Cancer Res., 1995, 55:259-66. Kawamori, T. et al, Cancer Res., 1999, 59:597-601) and in the multiple neoplasia (Min/+) mouse (Mahmood, N. N. et al, Carcinogenesis, 2000, 31:921-27), a genetic model of the human disease familial adenomatous polyposis. Curcumin acts as a scavenger of oxygen species such as hydroxyl radical, superoxide anion and singlet oxygen (Subramanian, M. et al, Mutat. Res., 1994, 311:249-55; Tonnesen, H. H. et al, Int. J. Pharm., 1992, 87:79-87; Reddy, A. C. P. et al, Mol. Cell Biochem, 1994, 137:1-8) and interferes with lipid peroxidation (Donatus, I. A., Biochem. Pharmacol., 1990, 39:1869-75; Sharma, S. C. et al, Biochem. Pharmacol., 1972, 21:1210-14). Curcumin suppresses a number of key elements in cellular signal induction pathways pertinent to growth, differentiation and malignant transformations. Among signaling events inhibited by curcumin are protein kinases (Liu, J. V. et al, Carcinogenesis, 1993, 14:857-61), c-Jun/AP-1 activation (Huang, T. S. et al, Proc. Natl. Acad. Sci., 1991, 88:5292-96), prostaglandin biosynthesis (Huang, M-T. et al, In L. W. Battenberg (ed.) Cancer Chemo prevention, CRC Press, Boca Raton, 1992, pp 375-91) and activity and expression of the enzyme cyclooxygenase-2 (Huang, M. T., et al, Cancer Res., 1991, 51:813-19; Zhang, F. et al, Carcinogenesis, 1999, 20:445-51). This latter property is probably mediated by the ability of curcumin to block activation of the transcription factor NF-κB at the level of the NF-κB inducing kinase/IKKα/β signalling complex (Plummer, S. et al, Oncogene, 1999, 18:6013-20).
Curcumin directly inhibits cyclooxygenase-2 and also inhibits the transcription of the gene responsible for its production. Cyclooxygenases (COX) catalyze the synthesis of prostaglandins (PGs) from arachidonic acid. There are two isoforms of COX, designated COX-1 and COX-2. COX-1 is expressed constitutively in most tissues and appears to be responsible for housekeeping functions (Funk, C. D. et al, FASEB J., 1991, 5:2304-12) while COX-2 is not detectable in most normal tissues but is induced by oncogenes, growth factors, carcinogens and tumor promoters (Subbaramiah, K. et al, 1996, Cancer Res., 1996, 56:4424-29; DuBois, R. N. et al, J. Clin. Invest., 1994, 93:493-98; Kelley, D. J. et al, Carcinogenesis, 1997, 18:795-99). Several different mechanisms account for the link between COX-2 activity and carcinogenesis.
Curcumin is not simply an alternative to non-steroidal anti-inflammatory drugs (NSAIDS), which also have anti-inflammatory and cancer chemopreventive properties. This is so because COX is a bifunctional enzyme with cyclooxygenase and peroxidase activities. Aside from being important for PG synthesis, the peroxidase function contributes to the activation of procarcinogens. Therefore, the failure of NSAIDS to inhibit the peroxidase function of COX potentially limits their effectiveness as anticancer agents. Curcumin, in contrast, down-regulates levels of COX-2 and thereby decreases both the cyclooxygenase and peroxidase activities of the enzyme.
Curcumin is among the few agents to block both the COX and LOX (lipoxygenase) pathways of inflammation and carcinogenesis by directly modulating arachidonic acid metabolism. In a study to evaluate the effect of curcumin on the metabolism and action of arachidonic acid in mouse epidermis, it was found that topical application of curcumin inhibited arachidonic acid-induced ear inflammation in mice (Huang, M. T., et al Cancer Res., 1988, 48:5941-46; 1991, 51:813-19). Curcumin (10 μM) inhibited the conversion of arachidonic acid to 5- and 8-hydroxyeicosatetraenoic acid by 60% and 51%, respectively (LOX pathway) and the metabolism to PGE2, PGF2αand PGD2 by 70%, 64% and 73%, respectively (COX pathway). In another study, dietary administration of 0.2% curcumin to rats inhibited azoxymethane-induced colon carcinogenesis and decreased colonic and tumor phospholipase A2, phospholipase CγI, and PGE2 levels (Rao, C. V. et al., Cancer Res., 1995, 55:259-66). In this study, dietary curcumin also decreased enzyme activity in the colonic mucosa and tumors for the formation of PGE2, PGF2α, PGD2, 6-keto-PGF2αand thromboxane B2 via the COX system and production of 5(S)-, 8(S)-, 12(S)-, and 15(S)-hydroxy-eicosatetraenoic acid via the LOX pathway was also inhibited.
Despite this impressive array of beneficial bioactivities, the bioavailability of curcumin in animals and man remains low. In rodents, curcumin demonstrates poor systemic bioavailability after p.o. dosing (Ireson, C. R. et al, Cancer Res., 2001, 41:1058-64) which may be related to its inadequate absorption and fast metabolism. Curcumin bioavailability may also be poor in humans as seen from the results of a recent pilot study of a standardized turmeric extract in colorectal cancer patients (Sharma, R. A. et al, Clin. Cancer Res., 2001, 7:1834-1900). Indirect evidence suggests that curcumin is metabolized in the intestinal tract. Curcumin undergoes metabolic O-conjugation to curcumin glucuronide and curcumin sulfate and bioreduction to tetrahydrocurcumin, hexahydrocurcumin and hexahydrocurcuminol in rats and mice in vivo (Pan, M. H. et al, Drug Metabol. Dispos., 1999, 27:486-94; Asai, A., et al, Life Sci., 2000, 67:2785-93) in suspensions of human and rat hepatocytes (Ireson et al, loc. cit) and in human and rat intestine (Ireson, C. R. et al, Cancer Epidemiol. Biomark. Prev., 2002, 11:105-11). Metabolic conjugation and reduction of curcumin was more in human than in rat intestinal tissue. It has been suggested that the intestinal tract plays an important role in the metabolic disposition of curcumin. This is based predominantly on experiments in which [3H] labeled curcumin was incubated with inverted rat gut sacs (Ravindranath, V. and Chandrasekhara, N., Toxicology, 1981, 20:251-57). This was later confirmed in intestinal fractions from humans and rats. Intestinal mucosa, as well as liver and kidney tissue from the rat, can glucurodinate and sulfate curcumin, as judged by the analysis of differential amounts of curcumin present before and after treatment of tissue extracts with conjugate-hydrolyzing enzymes (Asai et al, loc cit). Thus, gut metabolism contributes substantially to the overall metabolic yield generated from curcumin in vivo. In human intestinal fractions, conjugation with activated sulfuric or glucuronic acids was much more abundant, whereas conjugation in human hepatic tissues was less extensive, than in the rat tissues (Ireson, C. R., et al, Cancer Epidemiol. Biomark. Prev., 2002, 11:105-11).
Although p.o. administered curcumin has poor bioavailability and only low or non-measurable blood levels were observed (Perkins, S. et al, Cancer Epidemiol. Biomark. Prev., 2002, 11:535-40), this route of administration inhibits chemically induced skin and liver carcinogenesis (Limtrakul, P., et al, Cancer Lett., 1997, 116:197-203; Chiang, S. E. et al, Carcinogenesis, 2000, 21:331-35). Oral administration of curcumin also inhibits the initiation of radiation-induced mammary and pituitary tumors (Inano, H. et al, Carcinogenesis, 2000, 21:1835-41; Int. J. Radiat. Oncol. Biol. Phys., 2002, 52:212-23; ibid, 2002, 53:735-43). Similarly, in a study to assess the curcumin levels in the colorectum, a daily dose of 3.6 g curcumin achieves pharmacologically effective levels in the colorectum with negligible distribution of curcumin outside the gut (Garcea, G. et al, Cancer Epidemiol. Biomark. Prev., 2005, 14:120-25).
Earlier Shobha et al (Shobha et al, Planta Med., 1998, 64:353-56) had observed that administering piperine along with curcumin enhances the bioavailability of curcumin. However, the level of enhancement was only modest and no curcumin could be detected after 3 hours even when supplemented with piperine.
Although some questions remain unanswered regarding the pharmacokinetics of curcumin in humans, there is no denying the fact that considerable proportion of ingested curcumin is excreted through feces and at least about one-half of absorbed curcumin is metabolized. The quantity of curcumin that reaches tissues outside the gut is probably pharmacologically insignificant. Several studies have failed to demonstrate the positive invitro results with curcumin in invivo animal and human studies due to lack of absorption of curcumin after oral administration. To provide the clinical benefits, curcumin must be absorbed from its oral route of administration at a suitable rate, be distributed in adequate concentration in the blood and remain in the system for a sufficient period at an effective concentration level.
Both turmeric and curcumin are known for their antioxidant and anti-inflammatory activities, and may play roles in preventing atherosclerosis and cancer. Pharmacologically, turmeric has also been found to be a stimulant, a tonic, a carminative, and an anti-helmintic (Saleheen D, A. S. A., Ashfaq K, Siddiqui A A, Agha A, Yasinzai M M, Latent activity of Curcumin and their activity against Leishmaniasis in vitro. Biol Pharm Bull, 2002. 25: p. 386-9; Koide T, N. M., Ogihara Y, Yabu Y, Ohta N, Leishmanicidal effect of curcumin in vitro. Biol Pharm Bull, 2002. 25: p. 131-3; Gomes Dde C, A. L. V., deLIMA M E, LEON 11 and Araujo C A, Synthetic derivatives of curcumin and their activity against Leishmania amazonensis. Arzneimittelforschung, 2002. 52: p. 120-4). Curcumin has antibacterial and antifungal, anti-inflammatory, anti-allergic and wound healing properties (N, C.-W., Safety and anti-inflammatory activity of curcumin: a component of tumeric (Curcuma longa). J Altern Complement Med., 2003. 9(1): p. 161-8).
A large proportion of head and neck cancer develop from pre-existing oral premalignant lesions. Through several strategies have been attempted to treat these premalignant lesions, none so far has been found to be fruitful. This includes surgical excision, which has a relapse rate of about 40% and various chemopreventive agents. These are retinoid (vitamin-A), beta-carotine, Ketorolac (anti-inflammatory agent) and cetuximab (anti-EGFR therapy). Most of these agents although has demonstrated initial response, the lesions often relapsed upon cessation of the therapy. In addition, these treatments had various degrees of toxicities.
Head and neck squamous cell carcinoma (HNSCC) is the sixth most common cancer in the world and leading cancer in India. The age standardized incidence (ASR) range from 6.5 per 100,000 in Bangalore to 15.9 per 100,000 in the state of Kerala. While HNSCC account for 3% of all new cancer cases and 2% of cancer deaths in the United States in 1999, in India, it accounts for 30% of all cancers. Similarly, the incidence of oral leukoplakia in this population is also high. Case-control and cohort studies have established that this high incidence is due to widespread habit of tobacco use and alcohol exposure (Hashibe, M., Sankaranarayanan, R, Thomas, G, Kuruvilla, B, Mathew, B, Somanathan, T, Parkin D M, Zhang, Z F, Alcohol drinking, body mass index and the risk of oral leukoplakia in an Indian population. Int J Cancer, 2000. 88(1): p. 129-34). Tobacco is mostly consumed as smokeless tobacco in the form of pan. The incidence of p53 expression in premalignant lesion was reported as 55% (15/27) and that in the oral squamous cell carcinoma as 75% (24/32), where as normal epithelium did not show positive p53 expression (0/24) (Kaur J. et al, Overexpression of p53 protein in betel and tobacco related human oral dysplasia and squamous-cell carcinoma in India. Int. J. Cancer., 1994. 58(3): p. 340-5).
The malignant transformation rate of oral premalignant lesions from is about 8-36%, reported to be similar to that in other parts of the world (Gupta P C, Leukoplakia and incidence of oral cancer. J Oral Pathol, 1989. 18(1): p. 17). In a landmark primary prevention study of oral cancer (Gupta P C, et al., Intervention study for primary prevention of oral cancer among 36000 Indian tobacco users. Lancet, 1986. 1(8492): p. 1235-9) 36,471 subjects from Kerala, Andhra Pradesh and Gujarat were followed for 5 years. The follow up rate was 97%. Smoke cessation program was introduced in the interventional group. 5-year age adjusted incidence rate (per 100,000) of leukoplakia was 11.4 in the interventional group versus 47.8 among men and 5.8 versus 33.0 among women.
Silverman and his coworkers (Silverman Sol. Bilimoria K F. Bhargava K. Mani N J. Shah R A, Cytologic, histologic and clinical correlations of precancerous and cancerous oral lesions in 57,518 industrial workers of Gujarat, India. Acta Cytologica., 1977. 21(2): p. 196-8) screened a group of 57,518 industrial workers in India for oral cancer and pre cancer lesions. Fifty-one oral cancers were diagnosed (0.18%). In a follow up study of the same cohort identified 6,718 subjects with oral leukoplakia. After 2 years 4762 (71%) were reexamined Six (0.13%) individuals with leukoplakia developed oral cancer. This incidence of malignant transformation (63/100,000 per year), was similar to that reported from the US population. During the two year follow up period 57.3% lesions remained unchanged, 31.6% disappeared, and 11% had altered appearance (Silverman S. Bhargava K. Smith L W. Malaowalla A M. Malignant transformation and natural history of oral leukoplakia in 57, i.w.o.G., India., Malignant transformation and natural history of oral leukoplakia in 57,518 industrial workers of Gujarat, India. Cancer, Boone J Cell Biochem Suppl, 1976, 1992. 38(4): p. 1790-5, 23-6)
Within the upper aero digestive tract mucosa of patients at risk for tobacco related cancers, a well-defined precursor oral premalignant lesion (OPL) for oral and pharyngeal carcinoma has been defined, i.e. oral mucosal dysplasia (Pindborg J J, Oral Cancer and Precancer. Bristol: John Wright and Sons, 1980; Lippman, S. M. and W. K. Hong, Molecular markers of the risk of oral cancer. N Engl J Med, 2001. 344(17): p. 1323-6). Clinically these dysplastic lesions appear as white (leukoplakia) and red (erythroplakia) patches or as mixed (speckled leukoplakia) lesions. These lesions have variable malignant transformation potential. Oral and pharyngeal mucosa lesions transform into invasive tumors through well defined histological stages of hyperplasia, dysplasia, carcinoma in situ and invasive squamous cell carcinoma. The genetic changes associated with the histopathologic progression to upper aerodigestive squamous cell carcinoma has also has been established and a genetic carcinogenesis model has been proposed including early loss of heterozygosity (LOH) for tumor suppressor genes and later activation of protooncogenes (Slaughter D L, Southwick H W, and Smejkal W, “Field Cancerization” in oral stratified squamous epithelium: clinical implications of multicentric origin. Cancer Causes Control, 1953. 6: p. 963-8).
Prospective studies of subjects with OPLs revealed a significant incidence of malignant transformation to cancer depending primarily upon the presence of dysplasia. In the largest U.S. series consisting of 257 untreated oral leukoplakia subjects, Silverman et al., determined the malignant transformation rate at 8 years was 17.5%, however, the rate rose to 36.4% for those with dysplasia. None of the dysplastic lesions improved spontaneously. A large study by Silverman of Indian workers with OPL showed similar findings (Silverman Sol. Bilimoria K F. Bhargava K. Mani N J. Shah R A, Cytologic, histologic and clinical correlations of precancerous and cancerous oral lesions in 57,518 industrial workers of Gujarat, India. Acta Cytologica., 1977. 21(2): p. 196-8).
HNSCC results from a multi-step carcinogenesis process, which occurs over large areas of the upper aerodigestive tract epithelium exposed to carcinogens. This condemned mucosa contains multiple transformed clones that can develop into new primary tumors at a rate of 30% over five years. This process is called “field cancerization” (Strong M S, I. J., Vaughan C W, Field cancerization in the aerodigestive tract—its etiology, manifestation, and significance. J Otolaryngol, 1984. 13(1): p. 1-6; Pandey, M., Thomas, G., Somanathan, T., Sankaranarayanan, R., Abraham, E. K., Jacob, B. J., and Mathew, B., Evaluation of surgical excision of non-homogeneous oral leukoplakia in a screening intervention trial, Kerala, India. Oral Oncol, 2001. 37: p. 103-9). These patients harbor multifocal, metachronous, premalignant lesions. Currently there are no effective means of treating these lesions. Excision often leads to relapse. Although chemoprevention with retinoids has demonstrated proof-of-principle that this may be a potential approach to prevent oral cancer, the poor compliance and toxicity profile made this an ineffective.
The standard of care for OPLs is observation or removal. If the area is extensively involved, or multiplicity prohibits excision, the only alternative is close observation. The recurrence rate after excision of leukoplakia is 35% (P, N. P., Oral Oncol, 1997; Pandey, M., Thomas, G., Somanathan, T., Sankaranarayanan, R., Abraham, E. K., Jacob, B. J., and Mathew, B., Evaluation of surgical excision of non-homogeneous oral leukoplakia in a screening intervention trial, Kerala, India. Oral Oncol, 2001. 37: p. 103-9). Repeated surgical excisions can be associated with scarring and poor functional outcome.
Chemoprevention potential of several drugs has been investigated in the past with limited success. Following description summarize the outcome of these clinical trials.
Despite many clinical trials with retinoids, the narrow therapeutic window of these agents does not allow their safe routine use for these lesions (Lippman, S. M., Benner, S. E., and Hong, W. K., Cancer chemoprevention. J Clin Oncol, 1994. 12: p. 851-73).
A randomized, placebo-controlled, double blind trial evaluated the efficacy of 13-cis-retinoic acid in halting or reversing the development of OPLs. A total of 46 subjects were randomized to treatment with 13-cRA (1-2 mg/kg/day) or placebo for three months, with six further months of follow-up. Intolerable conjunctivitis and hypertriglyceridemia developed in 2 subjects receiving 2 mg/kg. Among the 24 13-cRA subjects, 2 had complete responses, 14 had partial responses, however, relapse occurred 2-3 months after end of 13-cRA therapy.
Another randomized, double-blind trial was designed to evaluate low-dose 13-cRA versus β-carotene in maintaining remission of oral premalignancy following induction therapy with high-dose 13-cRA. At the conclusion of induction, the rate of response was 55% (36 subjects), and the rate of stable disease was 35% (30 subjects). Of the 59 subjects included in the second phase, 53 were evaluable, of these, 22 in the 13-cRA group and 13 in the β-carotene group responded to maintenance therapy or continued to have stable lesions (92% vs. 45%).
A bio chemoprevention study employing a combination of 13-cRA, alpha-tocopherol and alpha-interferon was designed to address advanced premalignant lesions of the upper aerodigestive tract that are resistant to single agent retinoid intervention. At 6 months, 31 subjects were evaluable for response: 12 had a pathologic complete response, 7 partial response, and at 12 months, 8 complete and 7 partial responses.
Sulindac, a pan COX inhibitor is being tested for efficacy in the management of oral leukoplakia in a clinical trial at Amrita Institute of Medical Sciences (AIMS), Cochin. Sulindac has been shown to have anti-neoplastic effect against human oral squamous cell carcinoma in pre-clinical experiments. In addition, sulindac has been reported to be effective in preventing colon and esophagus tumors in several animal models. Recently, sulindac was shown to be safe and effective in humans for the prevention of polyps in familial adenomatous polyposis.
These studies were initiated because of the known dramatic over expression of Cyclooxygenase-2 (COX-2) in head and neck cancers and leukoplakia compared to normal tissue, and the known high levels of prostaglandins that may contribute to carcinogenesis in these patients.
The National Cancer Institute recently reported in the 2003 ASCO proceedings a negative trial of the topical NSAID Ketorolac for oral leukoplakia. The design of the proposed curcumin trial may allow efficacy not seen in the Ketorolac trial because of several differences. Curcumin is given systemically which might provide better drug distribution or availability, possible COX independent effects of these metabolites may lead to efficacy not apparent in the Ketorolac study.
Although the curcumin from turmeric can have wide medicinal use and biological effects have been suspected for over many decades, the challenge so far has been to increase bioavailability of the drug in blood, so that there will be tangible patient benefit.